1. Technical Field
This invention generally relates to a method and apparatus for injecting a gas hydrate into an environment of interest. More particularly, the invention concerns a method and apparatus for systemic oxygenation of hypoxemic blood by intravenous injection of an aqueous suspension of an oxygen clathrate hydrate.
2. Description Of Background Art
Oxygen administration by ventilation, even at a high inspired oxygen tension, may be ineffective in potentially reversible respiratory insufficiency in a clinical setting. Such conditions include adult respiratory distress syndrome, acute pulmonary edema, foreign body aspiration, pulmonary embolism, and respiratory distress syndrome of infancy. The problem of how to treat such conditions is compounded by the pulmonary toxicity that may result from prolonged exposure to relatively high inspired oxygen tensions.
Currently, the only potentially viable medical approach for systemic oxygenation of patients calls for use of an IVOX catheter. In such devices, gas exchange occurs at the interface of a membrane of multiple small tubules and blood in the inferior vena cava. Although the potential utility of such devices has been demonstrated clinically in a small number of patients, the large size of the catheter (7-10 mm diameter) which is inserted in the femoral vein, and the large surface area presented by the tubules may result in venous thrombosis and pulmonary embolism.
Entrapment of gases within liquids and solids is an important goal in many biologic and industrial processes. An example in biology is the uptake of oxygen by deoxyhemoglobin. In industry, gases are often used as foaming agents and are occasionally dissolved in fluids as reactants in chemical reactions. Various other examples include gasification of water for consumption, waste water management, and detoxification of hazardous wastes.
There are two commonly used means for gasification of liquid and solid media: 1) diffusion of gas into the media upon contact with its surface, and 2) injection of gas bubbles or of a gas-releasing precursor into the substance of the media. But diffusion is a relatively slow process. In order to inject bubbles into a medium to create a foam or to increase the surface area at a gas/medium interface, one can increase the rate of gas diffusion. However, it is often necessary to use a surfactant to reduce bubble surface tension so that sufficiently small and/or uniform bubbles can be produced. For certain applications, however, the presence of the surfactant is undesirable. Likewise, the breakdown products after liberation of a gas from a precursor may be problematic.
In the medical field, safe and effective oxygenation of hypoxemic blood (by intravascular injection of oxygen foam or an oxygen-liberating material) has not previously been achieved. Obstruction of capillaries by surfactant-stabilized foam, inadequate mixing with blood, or liberation of toxic breakdown byproducts (including toxic oxygen moieties) of an oxygen precursor would typically occur.
Accordingly, it is an object of the present invention to provide a means for injection of bubbles into a medium without the need for a surfactant.
It is a further object of the present invention to make the bubble size so small that mixing can be achieved rapidly, whereby excessive bubble coalescence and adherence between large numbers of bubbles do not occur.
In the case of oxygenation of blood, rather than using a foam, the need has arisen to inject a liquid suspension of microbubbles of a sonicated crystalloid solution without a surfactant or with a relatively biocompatible surfactant such as albumin. Certain techniques for encapsulating a hyperbaric gas for treatment with encapsulated gaseous precursors (such as microbubbles of oxygen) were disclosed in my co-pending U.S. patent application Ser. No. 655,078, which is herein incorporated by reference. The method disclosed therein includes the step of applying ultrasound energy to a probe disposed within an environment of a hyperbaric gas and an encapsulating material.
Previous approaches to the problems of delivering oxygen to hypoxemic blood, wherein a sonicated crystalloid solution is injected by a liquid suspension of microbubbles often produce an oxygen content which is insufficient to produce significant systemic oxygenation. For example, such approaches may only be capable of delivering less than 0.5 cc gas for each cc of the suspending fluid. To provide a desirable level of systemic oxygenation requires 50 to 300 cc oxygen/min. Therefore, it is another object of the present invention to provide a carrier for the gas-rich material, the carrier containing a volume of gas which is relatively small compared to the volume of gas delivered. Typically, a gas content of&gt;10 cc gas per gram of injectate is desirable.
Additionally, the need has arisen for a method which permits intravenous oxygenation with a relatively small catheter (&lt;2 mm). A commercially available ultrasound imaging guidance system allows rapid percutaneous insertion of catheters into the internal jugular vein, thereby permitting insertion of a suitable catheter into the right atrium or vena cavae within several minutes.